1. Field of the Invention
The present invention relates to capillary holders useful in capillary electrophoresis instruments. More particularly, the present invention involves holders which secure and protect capillaries during their installation and subsequent use in electrophoretic separation.
2. Description of Related Art
A detailed description of capillary electrophoresis is provided in U.S. Pat. No. 4,675,300 issued to Zare et al. and U.S. Pat. No. 5,484,571, issued to Pentoney, Jr. et al., both of which are incorporated herein by reference.
Capillary electrophoresis is commonly used in analytical and biomedical research for the rapid separation and analysis of charged species including synthetic polynucleotides, DNA sequencing fragments, DNA restriction fragments, amino acids, optical isomers of dansyl amino acids, and the separation of proteins, viruses and bacteria.
Capillary electrophoresis typically utilizes an electrokinetic differential to separate and transport components of a sample inside a capillary. A variety of detectors are available for detecting the sample components including uv-visible detection, electrochemical detection and more recently laser induced fluorescence detection which involves labeling sample components with a fluorescent tag, such as, e.g., fluorescein.
Laser induced fluorescence utilizes one or more lasers to illuminate a window section of the capillary. This illumination results in the laser excitation of fluorescent material within the window section and causes the fluorescent material to emit radiation. These emission are collected by a collection device and sent to a detector which converts the emission energy to a usable signal for analysis. Many capillary systems will accommodate multiple fluorescent labels each of which has its own emission characteristic or emission wavelength. Accordingly, information about the nature of many sample components can be obtained by collecting the fluorescent emissions from the components and noting the wavelength and/or intensity of the emission.
In order to increase sample throughput, recent capillary electrophoresis system designs incorporate multiple capillaries in a single system. Many of these systems use only a single detector to interrogate and detect sample components in multiple capillaries. In order to handle and install multiple capillaries, great care must be taken to protect the capillaries and guide them into position. Moreover, since it is desirable to collect as much of the fluorescent emissions as possible to maximize the detection sensitivity, proper alignment between a laser or the radiation source, each capillary window section, and the fluorescent emissions collector is critical. When multiple capillary systems are utilized, installing the capillaries and properly aligning each capillary window can be very challenging.
Typical capillaries are in a tubular configuration having a wall thickness of between 25-500 microns. The wall is made of glass, fused silica or an organic material such as polytetrafluoroethylene, commonly sold under the trademark "Teflon" by DuPont. In order to protect fused silica capillaries the wall is strengthened with a polyimide coating. However, the polyimide coating is removed from the capillary window section to allow light to pass. resulting in a relatively weak and fragile capillary window section. Thus a major consideration in installing and removing capillaries is to avoid damaging the window and breaking the capillaries.
The concern for protecting and securing capillaries is especially critical for high volume users. Although capillaries have long useful lives, they are not indefinite and they need to be removed and replaced. The primary reason for replacing capillaries is the degradation and the loss of interior wall coatings which are introduced on the walls to reduce or eliminate electroosmotic flow during the electrophoretic process. Moreover, many electrophoretic separations require that the separation medium be pumped from each capillary and replaced by pumping in fresh medium after each separation. Thus, the capillaries need to be secured during the separation medium replacement process and they need to be protected when they are replaced and aligned.
One way of retaining a single capillary is provided in U.S. Pat. No. 5,484,571. With reference to FIG. 4 of U.S. Pat. No. 5,484,571, the capillary can be inserted through and retained by small holes in the sides of a parabolic reflector. However, this system is not entirely satisfactory since the fragile capillary window section is not protected during the capillary coating process or its installation into the parabolic reflector. Additionally, in this system the window section can be difficult to position properly in front of the focal spot of the light source and it holds only one capillary.
Accordingly, there is a need for a device that supports the window section of the capillary during its installation into an electrophoretic system, and that facilitates the correct alignment between the window section and the focal spot of the light source. Further, there is a need for a device which retains a plurality of capillaries and allows for the replacement of the support medium within the capillary without removing the capillary from the electrophoretic system.